1. Field of the Invention
This invention relates to a de-centered optical system and an optical apparatus such as an image observation apparatus, an image projection apparatus, an image taking apparatus and so on, in which a de-centered optical system is provided as an optical system.
2. Description of the Related Art
Laid open Japanese Patent publications Hei 8-313829 and Hei 9-113841(counterpart U.S. Pat. No. 5,818,641) disclose an image observation apparatus of the face mounting type using a de-centered optical system as an ocular optical system thereof to guide light from an image display device to an observer""s eye. The ocular optical system has a prism having three optical surfaces of a first, second and third optical surface arranged in this order inversely along the light traveling direction from the display device. The first optical surface has refracting and internal reflecting functions, the second optical surface is a reflecting surface positioned opposite to the first optical surface and has a positive optical power and is de-centered from the optical axis of the ocular optical system, and the third optical surface is a refracting surface for refracting light refracted by the first optical surface and reflected by the second optical surface and further reflected by the first optical surface.
Laid open Japanese Patent publication Hei 9-146037(counterpart U.S. Pat. No. 5,745,295) discloses an ocular optical system having a prism, similar to the above described one, as a first optical element and a positive lens disposed on the observer""s eye side of the prism as a second optical element.
Laid open Japanese Patent publication Hei 6-347708 discloses an image observation apparatus or an image projection apparatus, in which the size of an image appearing on the image display device varies in dependence on the color thereof, in order to cancel chromatic aberrations of an optical system used in the apparatus.
Among above described known apparatus, the one disclosed in laid open Japanese Patent publication Hei 9-146037 has realized both a wide angle of view and good correction of aberration. However, as the second optical element is a positive lens having a concave surface on its observer""s eye side, the optical system is not sufficiently miniaturized. Moreover the positive power of the second optical element produces a large amount of chromatic aberration of magnification.
This invention will provide an optical system having a wide angle of view which is small in size and suitable for an ocular optical system of an image observation apparatus to observe an image appearing on an image display device or a projection optical system of an image projection apparatus to project an image appearing on an image display device. This optical system can be used as an image taking optical system by setting the light traveling direction inversely to that of the case where it is used as an ocular optical system.
The de-centered optical system according to this invention comprises a first optical element and a second optical element. The first optical element has at least three optical surfaces, i.e., first, second and third optical surfaces, and the portion surrounded by these three surfaces is made of a medium having a refractive index of more than 1. The first optical surface has a function of refraction and reflection of light inside the optical element. The second optical surface is disposed on a de-centered position from an optical axis that is a center line of a light bundle and shaped as a curved surface having a positive optical power when reflecting light inside the optical element. The third optical surface has a function of refraction. These three optical surfaces are so configured that when light enters into the first optical element from the first optical surface, the light that entered from the first optical surface is reflected by the second optical surface inside the optical element in an oblique direction toward an area on the first optical surface not completely overlapping the entrance area of the first optical surface, is reflected by the first optical surface, and then exits from the third optical surface. The second optical element is disposed outside in front of the first optical surface of the first optical element. The surface of the second optical element disposed furthest from the first optical element is a convex surface and has a positive optical power and produces a chromatic aberration of magnification.
When light travels through the de-centered optical system in the direction from the second optical element toward the first optical element, the size of an image formed by the de-centered optical system in the blue light wavelength region is larger than that formed in the green wavelength region, and that formed in green wavelength region is larger than that formed in the red wavelength region.
When the de-centered optical system is used as an ocular optical system of an image observation apparatus, an image display device is disposed outside the third optical surface of the first optical element and an observer""s eye is positioned outside the second optical element. The light emanated from the image display device enters the first optical element through the third optical surface, is reflected by the first optical surface, then reflected by the second optical surface, and exits the first optical element through the first optical surface. Then the light goes through the second optical element and reaches the observer""s eye.
When the de-centered optical system is used as an image taking optical system, external light coming from an object is transmitted through the second optical element and enters the first optical element from the first optical surface. The light is reflected by the second optical surface, reflected by the first optical surface, and then exits the first optical element from the third surface and forms an image of the object. In this case, it is preferable to place the pupil of the image taking optical system at the entrance side of the second optical element. Therefore, whether it is used an ocular optical system or an image taking optical system, the first and second optical elements are placed between the pupil, that is, an observer""s pupil (eye point) or the pupil of the image taking optical system (an aperture stop or a conjugate thereof) and the image surface, that is, an image display surface of the display device or the surface on which an image of an object is formed. Among the three surfaces of the first optical element, the first optical surface is closest to the pupil and the third optical surface is closest to the image surface.
Other features and advantages of this invention will become apparent from the following detailed description of the examples when taken in conjunction with the accompanying drawings and appended claims.